Great shelling on Cape San Blas. St. George Island and Mexico beach arent' bad either.
Like Suma mentioned, there are plenty of shells year around at these locations. There is a park on the eastern side of Mexico beach before you come to Wind Mark Resort that allows you to wander the shores. I`ve found many shells here. You can also follow Cape San Blas road out the St Joe Peninsula State Park and find plenty of shells along the shoreline. The further east you travel, the more plentiful the shells become.